'77 Jeep Wagoneer Gas Tank
Removal and Installation

Table of Contents

Rational

Ever since I've owned my '77 Jeep Wagoneer, the fuel
level gauge has been inaccurate. It's pretty close at empty, but when
you fill the tank, it only registers 3/4 full on the gas gauge. I know
the tank is full because I've put over 19 gallons into the 22-gallon
tank. This was annoying, but as long as the empty mark was accurate,
it really wasn't worth losing any sleep over.

As we started driving the Jeep on more long trips after the
engine rebuild, we noticed that you could smell
gas fumes inside the cabin after a while. You couldn't smell them
if you stuck your head under the hood, so I assumed that they were
likely caused by a problem with the fuel tank -- perhaps by cracked
rubber hoses.

Unlike the CJ's and California FSJ's of that era, which used a fuel
vapor canister in the engine bay to vent gas tank fumes into the
carb's air intake, the 49-state FSJ's in 1977 simply vented the gas
tank fumes into the atmosphere via a vented gas cap. It was suggested
that perhaps my old (non-stock) cap was non-venting or had a clogged
vent, so I bought a new, stock-style cap. No change in the fumes, but
it looks nicer than the previous one.

Removal

The fumes were starting to get annoying, so I decided it was time to
drop the tank and see what I found. You can do this job by yourself,
but it'll go a lot faster if you have a second person that knows what
they're doing. The first step was to run the tank as close to empty
as I could, since gas weighs about 6nbsp;lbs per gallon, and the tank
is rather awkward even when empty. When the engine stalled on my wife
about six blocks from home, I figured that was close enough. :-)

Note that the '70's FSJ's gas tanks are a different style than the
'80's tanks, so the instructions here are only vaguely accurate for
1980 and newer Wagoneers, Cherokees, and Grand Cherokees. I'm not
sure how far back this tank style goes, but I do know that it was
used from 1974-79. For more instructions on this era and the newer
FSJ's, see the IFSJA
tech section.

Removing the tank is reasonably straightforward, but it still took me
about 2.5 hours and left me rather sore. Even though my Wag has about
2-3" of suspension lift, I found it helpful to jack up the driver side
of the Jeep to gain a little more clearance. How high you can raise it
depends on how high your floor jacks will reach when it comes time to
support the tank. I raised my left side frame about 3", which put it
about 5-6" higher than a stock Wag would sit. Be sure you block the
tires, because you'll have to disconnect the emergency brake later on.

The first step for me was to remove skid plate. This plate exists on
both my '77 Wag and my '78 parts Wag, but it's not mentioned in my TSM,
so you might be able to skip this step if you don't have one. The skid
plate isn't all that heavy, so I supported it with one floor jack while
I removed the dozen or so bolts that held it in place. To aid in this step,
I recommend spraying all the bolts/nuts with PB Blaster before you
begin. The skid plate mounts are completely separate from the gas tank
mounts, so you don't have to worry about the gas tank falling on you
just yet. There are six or so bolts (9/16" heads) along the frame rail,
which gave me the most trouble because they're exposed to the most road
grime. There are also two bolts along front edge about half way up,
two more at the top of the bracket half way back on the passenger side,
and two more at the back end of the tank near the frame rail.

Once the bolts are removed and the skid plate is mostly lowered, you'll
find the tricky part. The emergency brake cable runs through a hole in
the bottom of the skid plate as it passes from the tensioner to the frame
rail. In order to remove it, you have to remove the "ball" end of the
cable from the metal clip that connects it to the rear portion of the
cable. This clip is located inside the frame's C-channel about half
way back along the gas tank. To remove it, I had to free up some slack
in the cable by disconnecting the guide that holds it to the frame up
where it first contacts the frame. This guide is held in place by a
pin outside the frame with a cotter pin through it. Once you've got
some slack, you should be able to move the ball at the end of the cable
toward the center of the metal clip and then out through a hole in the
clip. On my clip, the hole wasn't large enough, so I had to pry it open
a bit farther using vice grips and a small pry bar. With the cable free,
you can thread it through the hole in the skid plate and then get the
skid plate out of your way.

Now it's time to remove the tank itself. The tank is held in place by
two bolts through a flange on the front edge, a strap under the middle
of the tank that's supported by one bolt at each end, and a bracket at
the left rear corner of the tank. Begin by squirting down both the
bolt and nut end of each of these bolts with PB Blaster. I'd
recommend you then loosen each of them part way before you clutter up
the work area with jacks to support the tank. It'll make the job of
breaking them free easier. I'd
recommend removing the bolts holding the center strap first, since they
gave me the most trouble and were the hardest to access. On my '77, the
bolt heads (facing downward) were 5/8" and the nuts (facing upward with
very little clearance) were 11/16". The bolt by the frame required about
6" of socket extensions and a wobble joint, plus a box-end wrench for the
nut on top of the frame. The bolts at the front and rear of the tank have
1/2" nuts and bolt heads. On the rear bracket, I recommend removing
the bracket at both ends (one bolt at the tank and two at the frame
rail). It makes removing the tank easier.

Now that you've broken loose all the bolts, it's time to disconnect
the four hoses from the tank. Probably not a bad idea to first remove
the gas filler cap to relieve any pressure that may exist inside the tank.
The two supply and return lines that
run to the engine bay are connected to the tank via short rubber hoses
near the frame rail at the front of the tank. Remove these hoses from
the steel lines. At the top of the rear of the tank is the fuel filler
hose. The filler vent hose is accessed from inside the left rear wheel
well, just in front of the tire. For now, just disconnect the rubber
hose from the metal line in the wheel well, and leave the rubber hose
connected to the fuel tank. It's not very long, and won't really get
in the way. This leaves only the sending unit wire connected to the
tank, but it's difficult to access until you drop the tank several inches.

Now it's time to support the tank so it doesn't fall on you when you
finish removing the bolts. I was able to do the job by myself with
two floor jacks. I used some 18-24" 2x6 boards on top of the jacks to
distribute the weight a little better and avoid denting the tank.
Once it's supported, you can finish removing the bolts.

The front of the tank is supported by a flange that rests on top of the
bracket, but the rear of the tank will start dropping as soon as you
remove the bolts and lower the jacks. Once the rear of the tank has
been lowered a few inches, you should be able to reach up and unplug
the wire that goes to the sending unit. It has one of those safety
clips on it that requires a screwdriver to pry free, so you probably
can't do this blind while the tank is fully raised. With the wire
disconnected, you're free to remove the tank. You'll have to move it
rearward a couple inches so the front flange can clear the mounting
bracket, but otherwise it should drop straight down. You can't move
it sideways right away thanks to the bracket to which the middle strap
attaches. If you're using two jacks, be careful to keep the tank mostly
level so that any fuel still in the tank doesn't run to the low end and
shift the weight suddenly.

This much of the removal process me about 2.5 hours by myself, the majority
of which was spent removing stubborn bolts. You might be able to do it
faster if your bolts are in good shape and you pay attention to the tips
mentioned above, some of which I learned the hard way.

R and R

There were several things that needed to be done while the tank was out.
Most obviously, I needed to replace all the short rubber fuel hoses that
connect the hard lines together. All of them were bad, but one almost
resembled the floor of a dry lake bed. The larger (roughly 1" ID) hose
that connects the tank to the filler vent line seemed to be in fine shape,
surprisingly. It had the factory hose clamps on it, so I'm guessing it's
original.

The most annoying hose problem I found was with the main filler tube.
The filler path on a '77 Wag is quite long. Originally, the filler goes
from the plastic filler mouth to a right-angle rubber hose that goes
from the quarter panel to under the body, where it connects to a metal
tube. This tube snakes past the spare tire, where it connects to another
rubber hose with two bends that eventually connects to the nipple on the
gas tank. The problem is that, after 26 years, both these rubber hoses
tend to go bad, and nobody makes them any more. The solution I came up
with was to have an exhaust shop bend a piece of aluminized exhaust pipe
in the shape that I needed, then splice that piece to the adjacent pieces
using short, straight lengths of rubber fuel hose. I already did this a
couple years ago with the elbow pipe closest to
the filler neck. When I inspected the hose by the gas tank, I found
that it had a bad crack and probably wouldn't last much longer, so I did
the same thing with it. Making the two bends at the proper angles proved
difficult for the shop, but they got it close for $20. I'm not wild
about trying to mate the end of my new pipe in mid-bend to the neck on
the fuel tank. Time will tell if it's sealed well enough.

Perhaps a better solution would have been to have a qualified shop weld
a longer, bent neck onto the tank that would allow the use of a 12"
straight rubber hose between the tank and the intermediate filler tube.
I didn't think of that until just as I was writing this, of course. :-(

As I was removing the fuel pickup to inspect it, I managed to break the
"sock" filter that fits over the end of the filler pickup. This is actually
a cylinder about 4.5" long and 7/8" in diameter. The outer surface is
two layers of plastic mesh. Inside is a foam-like tube that extends from
the metal tube down to the bottom of the tank. Nobody local carries
anything close to that, and I didn't feel like waiting to mail order one
(BJ's Off-Road sells them for $5).
The route I took was to ditch the in-tank filter and replace it with a 5"
length of 3/8" rubber fuel line. I positioned it so that it would just
barely touch the bottom of the tank (it goes in at a 45-degree angle).
To keep the hose from curling up, I sleeved it with a spring, which I
secured in place on the pickup tube/hose with some 20 awg wire. To
keep the end of the hose from mating flush with the floor of the tank and
causing a vacuum, I cut two small notches into the end of the hose -- just
in case.

You still need a filter prior to the fuel pump, though. I bought one of
those transparent in-line fuel filters (Purolator brand) with the replaceable
filter elements and placed it in the supply line to the fuel pump.
The filter is 4.5" long, 1-3/8" of which is hose nipples. I wanted
to place it just in front of the tank where the hard line starts to
run along the frame to the engine bay, but it was a good 2" too long
to fit there. I ended up putting it at the other end of that hard
line -- in the engine bay at the base of the rubber hose that supplies
the fuel pump. It's only a few inches from the front of the exhaust
manifold, which concerns me a little, but maybe it's no big deal.
We'll see. I'm having some occasional problems with vapor lock, so
I may have to do something different with that entire line anyway.

The most vexing part of this job was fixing the inaccurate fuel gauge
that I mentioned earlier. The gauge circuit works like this: in the
instrument cluster is a voltage adjustor that takes the 12V accessory
current and converts it to 5V, which it then provides to both the gas
gauge and temperature gauge. The other terminal of the gas gauge is
then grounded through the sending unit via a pink wire to the sending
unit and from the body of the sending unit through the tank to the frame.
The sending unit provides a variable amount of resistance in the ground
circuit, and this resistance is what changes the reading on the gauge.
According to the TSM, the sending unit provides 73 ohms at empty,
23 ohms at half tank, and 10 ohms at full (give or take 5%).
A short circuit will peg the gauge past full, while an open circuit (or
no accessory voltage) will peg the gauge past empty.

After pulling the sending unit (by using a screwdriver & BFH to drive
the lock ring counter-clockwise), I first cleaned up the electrical
connections by wire-brushing all the rust off the sending unit body
and the lock ring (be sure to use a brass wire wheel so it won't spark).
I then dug out my trusty, 4-yr-old, mildly-expensive multimeter to test
the resistance it provided. To my surprise, it provided somewhat less
than half the expected resistance across the board. After beating my
head against that for a while, I eventually determined that my multimeter
wasn't quite so trusty after all. A borrowed multimeter indicated that
the sending unit was functioning exactly to spec. I tested the circuit
between the gauge and sending unit by placing multimeter leads at the
chassis ground and at the pink wire terminal on the big, round connector
at the back of the instrument cluster. Same resistance, so there was no
significant resistance between the gauge and the sending unit.

The next step is to test the gauge by bypassing the sending unit and
putting a known resistance in-line with the ground circuit. I bought an
assortment pack of potentiometers (variable resistors) at Radio Shack,
one of which had a range of 0-100 ohms. I set the "pot" to a
known value with my borrowed multimeter, then turned the key to see
what the gauge registered. The gauge seemed to test out OK, so I'm
assuming the extra 5 ohms of resistance I was getting (enough to
force the needle from "full" down to 3/4 tank) must have come from a
bad ground between the sending unit and the frame. In addition to cleaning
up the sending unit housing and the bolt holes on the tank with a wire
brush, I also replaced two of the flat washers at the mounting bolts with
star-type lock washers, which will dig into the metal and provider a better
electrical connection.

I did run into two problems when testing the gauge that weren't documented
in the TSM. First, I tried to measure the resistance on the pot while
everything was hooked up, I got a much lower reading than I got if I
first disconnected the ground wire from the pot. I assume this is because
the temperature gauge (which is powered by the same 5V source as the gas
gauge) was providing a parallel resistance path to ground. Second, if I
tried to measure the pot's resistance with the key on, the values jumped
all over the board. I'm not quite sure what might be causing this, because
as I understand it, it means that there would have to be something in the
cluster that was shorting and then opening a circuit to ground several
times per second. This meant that I couldn't accurately adjust the pot
on the fly with the gauge active -- I had to disconnect the ground wire,
adjust the pot, then hook up the ground again. Not a big deal when using
alligator clips, but still something to beware of.

While I had the skid plate off, I also decided to coat it with some of
that rubberized, rattle-can undercoating (Mar-Hyde No. 4211). It cost
$4 a can, and I used a full can by putting a light coat on both sides of
the skid plate. It certainly won't hold up to any scrapes I may get
off-road, but it should do OK at preventing rust from road grime. Even
with the 26-yr-old factory paint job, the skid plate wasn't in that bad
a shape when I pulled it off. The spray can said that it deadens sound,
too, so this was mainly a test run to see how well I thought it might
work on the inside panels of my Wag. I think I'll keep looking. If you
buy this stuff, note that it takes well over an hour to cure (at least to
the point where newspaper won't stick if you flip it over).

Replacement

Re-installation of the tank is just the reverse of removal.

Before you slide the tank back under the Jeep, I recommend applying some
dielectric grease to both the sending unit stud and to the electrical
connector near the frame rail (where the white & pink wires meet).
I managed to do this while the tank was lifted up just 3" from the
floor pan, but it wasn't fun. Dielectric grease will help keep crud
out of the connection, which will help prevent rust, which will keep
unwanted resistance from skewing your gas gauge.

If you didn't label or otherwise keep track of which bolts & nuts went
where, you'll be kicking yourself, since there are several different
widths and lengths used, and it's hard to get a good look at most of
the holes. As a refresher, the pair of bolts, washers, and nuts at the
front of the tank are 5/16" (1/2" heads/nuts), as are the three bolts,
washers, and nuts that attach to the support bracket at the left rear
corner of the tank. The bolts, washers, and nuts that secure the strap
around the middle are 7/16" (5/8" heads, 11/16" nuts). The bolts (some
with nuts) that attach the skid plate are all 3/8" (9/16" heads/nuts).

In my case, this was complicated by the fact that I was trying to
retrofit a new fuel filter and filler hose that weren't there before.
See "R and R" for the stories. Lifting the tank
into place & connecting it was pretty straightforward. I had a helper
(my lovely assistant Stacy) push the jack under the big end of the
tank while I lifted it by hand (remember that there was still 2.5 gallons
of gas in my tank), but the rest of the process was easy to do by myself.

Note that it's easier to hook up the sending unit wire while the tank
is still several inches below its final resting place -- close enough
to reach, but still low enough that you can get your hand up above the
tank to do the job.

In order to help get a good electrical ground between the tank and the
frame, I replaced both the upper and lower washers on both of the bolts
at the front of the tank with double-star lock washers. These washers
dig into the metal as they're tightened, which will help maintain a good
electrical connection long after corrosion would have otherwise set in.

I recommend tightening all the attaching bolts most of the way, but
still allowing plenty of wiggle room at first. Once you've got everything
positioned properly and secured, then you can torque them down. A little
antiseize on the bolt threads wouldn't hurt, since all these bolts live
in a fairly corrosive environment. You do want to torque them down well
(especially the center strap), since you don't want your gas tank to
wiggle loose over time.

Recap

After having the Wag sit immobile in my driveway for a week doing all
this, after I finally drove to the gas station & filled it up, the gas
gauge STILL only goes up to 3/4 (or occasionally 7/8) tank when
the tank is really full. That's kind of frustrating, but I think the
problem must be in the gauge, not the sending unit. The empty mark
is still accurate, so I don't really care too much about it right now.

I haven't driven it enough yet to build up pressure in the gas tank,
so I don't know yet if I've fixed the problem of fumes in the cabin.
I'll know more in a couple days.

The transparent filter is nice to have. After driving only two blocks
to the gas station, I checked the filter, and was amazed at how much
crud had already collected there. Probably a result of having a slightly
longer pickup hose and maybe knocking loose some crud after moving the
tank around all week. Just to be safe, I bought some new elements for
the inline filter and a new replacement for the stock filter up by the
carb.

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